1. Field of the Invention
This invention relates to dredges, and in particular to a dredging apparatus for extracting bottom-dwelling shellfish such as clams, oysters, mussels, cockles, and crabs from a sea, lake, or riverbed and continuously transporting the extracted shellfish to a vessel towing the dredging apparatus.
2. Description of Related Art
(i) Introduction
Numerous attempts have been made to devise mechanical harvesters that move or that can be towed along the bottom of a body of water in order to harvest shellfish that live in colonies at the bottom. All of these devices seek to dredge shellfish such as clams, oysters, cockles, mussels, and/or crabs from the bottom of the body of water and either trap the shellfish for retrieval after the device is brought to the surface, or continuously transport the shellfish to the surface as the dredge is being towed along the bottom.
Common problems that the designers of these devices have attempted to solve include problems of efficiency, i.e., the relationship between power or effort expended and the amount of shellfish harvested, problems related to the cost and reliability of the device, which are often a function of complexity, and problems related to environmental damage caused by the device as it is towed across the sea, lake, or riverbed.
(ii) Mechanical and Hydraulic Dredges
The earliest attempts at large-scale shellfish harvesting devices undoubtedly date back to prehistoric times and most likely involved diggers or tongs dragged along the bottom for scooping shellfish into a collection cage or basket that could then be brought to the surface and emptied. More sophisticated but nevertheless fundamentally similar examples of dredges of this type are still being used and are disclosed, for example, in U.S. Pat. Nos. 4,827,635, 4,425,723, and 3,226,854. Such dredges have the advantage of simplicity, but are relatively inefficient because of inherent limitations in the effective of such mechanical dredging devices, and the need to repeatedly bring the dredges to the surface to be emptied.
As early as Greek times, use was being made of hydraulic device to harvest shellfish by using high pressure jets of water to slice a horizontal layer of sediment, followed by sifting of the loosened or liquified mass of shellfish-containing sediments to separate the shellfish from the sediments, and collection of the separate shellfish in a collection cage or basket. Such harvesters, which are also still in use, have the advantage of being able to dredge a relatively large area in less time than a purely mechanical harvester although they still require the collection cage or basket to be periodically brought to the surface for emptying.
(iii) Dredaes with Transport to the Surface
In order to avoid the need to periodically bring the dredge to the surface for removal of harvested shellfish, it has also been proposed to add conveyors that continously and automatically convey recovered shellfish to the surface, either in connection with a purely mechanical harvester or with one that uses a hydraulic digging action.
These conveyors can be either mechanical, hydraulic, or pneumatic, with mechanical conveyance systems being the most difficult to implement and as result, largely impractical. An example of a non-hydraulic dredge with a mechanical conveyor is nevertheless disclosed in U.S. Pat. No. 4,464,851, while U.S. Pat. Nos. 2,508,087, 3,462,858, and 3,521,386, disclose hydraulic dredges with mechanical conveyors in the form of conveyor belts or escalators.
In order to avoid the complexity of mechanical conveyance systems, it has been proposed to use a siphon effect to lift water in which shellfish have been entrained to the surface. Background examples of hydraulic shellfish transport arrangements are disclosed, for example, in U.S. Pat. Nos. 3,184,866 and 3,624,932.
U.S. Pat. No. 3,624,932 is of particular interest because it discloses an arrangement in which, like that of the present invention, a jet of water is directed across a layer of sediment to propel the sediment together with any shellfish onto a separating platform consisting of parallel blades that break up the sediments and clumps of shellfish, and that are spaced sufficiently to permit sediments and undersized shellfish to fall through, with any shellfish that make it up the platform being lifted into a conveyor by suction created in the lift pipe. Unlike the present invention, however, the arrangement disclosed in U.S. Pat. No. 3,624,932 requires a separate transport system powered by upwardly directed jets of air rather than water.
(iv) British Patent Publication No. 1,156,547
The closest prior art is disclosed in British Patent Publication No. 1,156,547. In the arrangement disclosed in this publication, dredging is carried out by directing pressurized water rearwardly relative to the direction of travel of the dredging apparatus so that the water jet sweeps sediments and shellfish towards a separator device and suction chamber. The suction chamber is connected to a trunk line through which shellfish that have been separated from the sediments are transported to the surface. The present invention also utilizes a water jet, separator, and suction chamber, but makes two key improvements relative to the apparatus disclosed in the British publication.
In the dredging apparatus disclosed in the British publication, as illustrated in FIGS. 1-4 herein, a sled 1 is arranged to be towed by a vessel 2 and cables 3,4 across the seabed 5. The sled 1 includes a digging blade 6 inclined forwardly and downwardly in the towing direction so as to extend below the runners or supports 7 for the main body of the sled. Facing the digging blade 6 is a digging water jet 8 arranged to direct a jet of water supplied by a pressure hose 9 in the direction of arrow A towards the digging blade for the purpose of loosening sediments and sweeping the sediments and shellfish residing therein past the blade to a separating device 10. Separating device 10 is in the form of a grid of bars 11 spaced above the runners immediately to the rear of the digging blade, and is designed to permit undersized shellfish, sediments, and other relatively small debris to fall past the bars and return to the seabed while facilitating movement of properly sized shellfish into suction chamber 12. A separating water jet 13 is also connected to the pressure hose 9 for facilitating separation of the shellfish, with "fish tail" suction chamber 12 being situated immediately to the rear of the separating device 10. Trunk line 14 is coupled to the suction chamber 12 for conveying shellfish that have passed the separating device 10 to the vessel 2 by means of a water jet 15 connected to the pressure hose 9 and extending into the trunk line 14. Water jet 15 creates a negative water pressure to cause water to be sucked from the suction chamber 12 into the trunk line 14 towards the vessel 2. At the surface, a channel screen 16 is mounted outboard of the of the vessel 2 so that the discharge from the trunk line pours into the channel screen, permitting the water to pass through and leaving shellfish for collection.
A first problem with the above-described apparatus is that while the separating bars 11 effectively separate out loose sediments and undersized shellfish, neither the knife blade nor the separating bars are capable of removing larger objects such as clumps of shellfish and debris from the stream of entrained shellfish before the larger objects are deposited in the suction chamber 13. As a result, it is possible for the suction chamber to become blocked, resulting in damage to the apparatus or, at best, costly delays while the apparatus is brought to the surface and unclogged.
A second problem with the apparatus described in the British publication and illustrated in FIGS. 1-4 is that the "fish tail" suction arrangement requires relatively large hydraulic pressure in order to create enough suction to transport shellfish to the surface, making the apparatus impractical for use on many fishing vessels or trawlers.
In addition, even if the suction chamber or trunk line of the apparatus shown in the British publication and in FIGS. 1-4 does not become clogged by debris that has made it passed the separating device, the presence of such extraneous material further increases the amount of pressure necessary to carry out the functions of digging, entrainment, and conveyance of the desired shellfish to the surface.
As a result of these problems, a need still exists for a dredging apparatus capable of extracting and continuously transporting shellfish to the surface, as in the British publication, and yet which more effectively separates out both larger and smaller debris before the debris enters the suction chamber, and which reduces power requirements for the water jet.